The present invention relates to hydrophilic films and methods for forming such film on the surface of a substrate made of material such as glass, metal, plastic, or the like.
Hitherto, for example, inorganic plate glass has been widely used for various articles such as window pane, mirror and eyeglass lenses, by the reason of its transparency and other good characteristics. When such article is used, for example, in a place of high temperature and high humidity, the dew condensation is caused on its surface, thereby to fog the same. In addition to this fogging problem, there has been another problem, for example, in the automotive outside mirror, to have many raindrops on its surface in the rainy weather. These problems interfere with the automotive rear view of the driver. Thus, there have been various proposals to provide the above-mentioned articles with antifogging property, water-drops preventive property, and durability. For example, there has been a proposal to form a hydrophilic film on the surface of a substrate such as glass plate, for the purpose of preventing fogging and the water-drops attachment thereto. There has been known for a long time a proposal of applying a surface active agent to the surface of such substrate, for achieving this purpose. For example, Japanese Patent Unexamined Publication JP-A-52-101680 discloses an antifogging agent for transparent article such as glass. This agent contains polyacrylic acid, a surface active agent, and a solvent which is water and/or alcohol. JP-A-55-154351 discloses a hydrophilic film formed on substrate. This film contains a phosphorus oxide and at least one of a molybdenum oxide and a tungsten oxide. JP-A-54-105120 discloses a method for producing an antifogging glass article by contacting an inorganic glass substrate containing P.sub.2 O.sub.5, with a P.sub.2 O.sub.5 -containing liquid or apor. Furthermore, JP-A-53-58492 discloses an antifogging agent containing at least one sulfonic-acid-type amphoteric surface active agent represented by a general formula disclosed therein, and at least one particular inorganic salt or acetate represented by a general formula disclosed therein. International Laid-open Publication WO96/29375 discloses a method of photocatalytically making the surface of a base material ultrahydrophilic. This method comprises the step of coating the surface with a layer containing photocatalytic semiconductor material such as titania and the step of photoexciting the photocatalytic material. In this publication, there is disclosed a photocatalytic coating formed on a substrate. This coating, which is superior in abrasion resistance, is composed of a mixture of titania and silica. There is further disclosed in this publication that such coating is formed on a substrate at first by preparing a sol mixture of an anatase-type titania sol and a colloidal silica sol, then by spraying the sol mixture onto the substrate, and then by baking the coated substrate. There is a demand for a hydrophilic film which has a long time duration in hydrophilicity.